The present invention relates to a method for controlling vehicle stability and more particularly relates to detecting and favorably influencing a defined driving situation in curves.
FIG. 1 shows a road surface 10 on which a vehicle 11 drives in the direction of the arrow v which represents a certain vehicle speed v. The road surface has an inner area 10i on which the inner wheels 12i of the vehicle travel and an outer area 10a on which the outer wheels 12o of the vehicle 11 travel. The separation is indicated by the dotted line (which is not intended to be the center line of a road). During cornering, the outer wheels 12o are usually subjected to greater load than the inner wheels 12i, because a rolling moment about the longitudinal axis of the vehicle is produced by the centrifugal force which is applied to the point of gravity of the vehicle situated above the road surface 10. The rolling moment must be compensated by an increased force on the outer wheels 12o. When braking in curves is so intensive that the anti-lock system (ABS) intervenes, the result is that due to the higher wheel load on the outside of a curve, the outer wheels 12o usually experience a less potential or, at most, equally potential, braking pressure decrease compared to the respectively associated inner wheels 12i. Nevertheless, swerving of the vehicle has been encountered in critical operating conditions with ABS in such a fashion that the vehicle turns inwardly in a curve.
An object of the present invention is to provide a method and a device for detecting the above critical condition and a method and a device for favorably influencing the yaw torque in the above situation.
The case may occur that the coefficient of friction xcexc between wheel 12 and road surface 10 on outside road surface portions 10o is lower than the coefficient of friction between the wheels 12i on the inside of a curve and the road surface inner side 10i. This may cause an outer wheel 12o to lock earlier than an inner wheel 12i so that, accordingly, the ABS reduces the brake pressure po for an outwardly disposed wheel 12o to a greater extent than for an inwardly disposed wheel 12i in order to prevent locking of the outer wheel. The result is that the brake force Fo on the outer wheel is lower than the brake force Fi on the inner wheel 12i. This generates a yaw torque MY about the vertical axis of the vehicle which, different from the conventional case, is not opposed to the vehicle rotation that is already provided, but turns in the same direction so that there is the risk of the vehicle, induced by the yaw torque MY, beginning to turn inwards into the curve due to the higher brake force.